Dear Sir,

It would be a pleasure to me to assist you in your researches, but I cannot give the desired information, as I have not specially attended to the embryology of the Vertebrata.¹ I strongly advise you to procure F. Balfour’s ‘Comparative Embryology’ in 2 volumes, which contains many original observations with full references to all that has been published on the subject, together with many illustrations.² I believe it to be the most valuable biological work which has been published for very many years. It will soon be translated into German, and I believe into French.

I have received your Essays, but just lately I have been so much engaged that I have not yet read them, but hope soon to do so.³

With respect to species, I quite agree with you that it is a serious evil to load our books with new names. Nevertheless the description, as a variety, of any constant difference, however small, between two forms, seems to me highly advisable.

As far as I can judge there is no rule about giving a new name to a form, excepting that of its differences being of an important nature. Whenever two forms are connected by intermediate varieties, it seems best to give the whole series a single name, however different the extremes may be; and I followed this plan in describing the Cirripedia.⁴

I am delighted at the revival of Science in Spain, and with all good wishes, I remain Dear | Sir Yours faithfully | Charles Darwin